Dental crown has a three-layer structure made of enamel, dentin, and pulp from outside to inside.
The enamel is a hard and highly insoluble layer mainly consisting of hydroxy apatite. However, once exposed to an acidic medium generated by glycolysis of food debris or the like by oral microbes, dissolution (de-calcification) of phosphate ions or calcium ions is promoted and white discoloration (white spots) occurs as an early symptom of dental caries, leading to so-called tooth decay.
As a means of preventing dental caries, fluoride application is known. Specifically, when teeth are treated with a fluoride ion source, hydroxy apatite is modified to fluoroapatite to give acid resistance. Further, it is well known that the treatment has a working effect of promoting formation (re-calcification) of hydroxyapatite by inhibiting dissolution (de-calcification) of phosphate ions or calcium ions, and for example, a treatment for preventing dental caries by application of a fluoride preparation containing a fluoride ion source on tooth surfaces is widely performed.
The preparation generally used as a fluoride preparation in the clinical practice of dentistry is an acidulated phosphate fluoride solution (herein below, abbreviated as “APF”). The working mechanism includes de-calcification of dental component by phosphate due to the acid phosphate property of APF, and the reaction between dissolved calcium ions and fluoride ions yields generation of calcium fluoride on tooth surfaces. However, the problems of APF treatment is that, when exposed to saliva, it is highly likely that the effect is not fully exhibited in an oral cavity. For such reasons, it needs to be maintained for four minutes in a state that the APF is applied on a tooth and intaking of food is prohibited for another 30 minutes or more after that. As such, it has a limit that a dentist may find it fairly troublesome to carryout and a burden on a patient is huge. It is also known that, as calcium fluoride precipitated on tooth surfaces is dissolved in saliva under a de-calcification (acidic) environment, the effect of promoting re-calcification is insufficient.
Composition for oral cavity containing a calcium ion source, a phosphate ion source, and calcium phosphate or the like as well as a fluoride ion source is reported. By simultaneously precipitating calcium fluoride and calcium phosphate on tooth surfaces, an effect of preventing dissolution in saliva under a de-calcification (acidic) environment and promoting re-calcification is expected.
In this connection, a preparation containing separate preparations of a fluoride ion source and a calcium ion source, that are admixed with each other at the time of use, is studied. For example, disclosed in patent document 1 is a product for re-calcification of tooth enamel comprising the first component (first liquid) containing water soluble calcium salts and the second component (second liquid) containing water soluble phosphate salt and water soluble fluoride salt, and it is specifically described that “ . . . applied . . . after mixing the first solution with the second solution” (Patent Document 1, page 9, lines 7-8). However, according to the patent document, it is intended to repeatedly carry out the cycle plural times, wherein a single cycle consists of treatment for 5 min and re-calcification for 60 min in saliva. Thus, there is no intention of obtaining microparticles precipitates during a short time (e.g., during dental treatment) as described in the present invention.
For the purpose of re-calcification of de-calcified enamel (dental caries at early stage), (1) a composition for generating calcium phosphate and calcium fluoride by mixing water soluble calcium salts of the first liquid with water soluble phosphate salt and water soluble fluoride salt of the second liquid on tooth surfaces (see for example, Patent Documents 1 to 3 and 8), and (2) a composition for generating calcium phosphate (see for example, Patent Document 9) are suggested.
Primary particles of calcium fluoride that are generated by mixing a fluoride ion source and a calcium ion source are disadvantageous in that they form secondary particles with large particle diameter by self-aggregation and may not easily adsorb on tooth surfaces. As such, a preparation added with a calcium fluoride inhibitor to delay the aggregation (see for example, Patent Document 2) or a composition for oral cavity allowing easy adsorption of calcium fluoride in microparticles state on tooth surfaces by mixing liquid A containing a compound for supplying a polyol phosphoric acid ion (specifically, calcium glycerophosphate) and a compound for supplying a monofluorophosphoric acid ion and liquid B containing sodium fluoride at the time of use to control aggregation rate is reported (see for example, Patent Document 3).
However, the former is problematic in that adsorption of fluoride on tooth surfaces is also inhibited by addition of calcium fluoride inhibitor and the latter is problematic in that stability of the solution containing calcium glycerophosphate ion and a compounds for supplying monofluorophosphoric acid ion remains unsatisfactory.
It is believed that glass ionomer cement, which is one type of dental cements, is expected for its activity of strengthening teeth by the fluoride contained as a glass component. Having superior biocompatibility, adhesiveness, and aesthetics, the glass ionomer cement is widely used for filling dentin cavity, a crown, an inlay or adhesion of a bridge or an orthodontic bracket. However, when it is in contact with water such as saliva in early hardening stage, the hardening reaction is inhibited and inferior physical properties are obtained in the end. Further, as there are drawbacks that the polished surface after hardening is coarse and film is thick so that feelings in tongue or aesthetics are poor, efforts are being made to improve them. As a method of improving coarseness of a polished surface or film thickness, use of powder for glass ionomer cement having specific gravity of 2.4 to 4.0, mean particle diameter of 0.02 to 4 μm, and BET specific surface area of 2.5 to 6.0 m2/g is suggested (see for example, Patent Document 4). However, as a specific example, only the powder with mean particle diameter of 2.0 to 2.2 μm and maximum particle diameter of 3.49 to 3.95 μm is disclosed. As described therein, the dental glass ionomer cement powder is the same as cement used in construction and it is first mixed with a liquid exclusive for cement and then used. Therefore, using itself as a liquid preparation is not supposed and examples therefor are not described either.
In clinical dentistry, it is reported that a severe pain is caused not only by dental caries but also by hypersensitivity. As explained above, a dental crown has a three-layer structure made of enamel, dentin and pulp, and dentinal tubules are extended all over the dentin. Although the dentinal tubules are generally covered by enamel, gum or the like, dentinal hypersensitivity is often caused when the dentinal tubules are opened for some reasons. For example, even when the dentinal tubules are exposed or opened by dental caries, use of a dental preparation containing abrasives, abrasion by bleaching carried out for aesthetic purpose, gum recession caused by aging, or the like, temporal but severe pain is caused by cold water or touch stimulation.
The mechanism of developing dentinal hypersensitivity is not fully elucidated. However, hydrodynamics is considered as a strong candidate. According to the hydrodynamics, various stimulations applied on dentin cause migration of fluid in dentinal tubules to excite the nerve fiber on the pulp side.
Thus, a treatment of inhibiting the migration of fluid in dentinal tubule is effective for improving dentinal hypersensitivity and it is reported that, as exemplified by coating a varnish containing a solvent and a resin for sealing a tooth, dentinal hypersensitivity is ameliorated or removed by sealing the dentinal tubules.
Examples of the methods for treating dentinal hypersensitivity are as follows; (1) treatment method for sealing dentinal tubules: in addition to potassium oxalate, a resin, strontium chloride, silver diamine fluoride, HY preparation, sodium fluoride solution, pasta added with sodium fluoride, a calcium hydroxide preparation, ion introduction or the like (Journal of Dentistry, August 1991, Vol. 34, No. 2), (2) treatment method for coating an exposed dentin: cement, paraform added dressing or the like, (3) restoration of a defective area: glass ionomer cement, adhesive resin or the like, (4) treatment by sedation of pulp nerves: administration of an anti-inflammatory pain-relieving agent, irradiation of soft laser or the like, and (5) pulp extraction.
Among them, adhesive resin or the glass ionomer cement having adhesiveness for teeth of (3) forms a strong film that is very difficult to remove. Thus, it is not appropriate for a case in which periodontal regeneration therapy on root canal surface may be applied.
Only a temporal effect is obtained from (4), and according to (5), the pulp, that is, nerves, is completely removed and blood vessels are removed together with nerves, and as a result, teeth are sacrificed even though the pains are completely eliminated.
The above (1) and (2) are a therapeutic method which does not involve a sacrifice of pulp or teeth, and therefore are appropriate for a case in which no major defect exists. However, care should be taken when choosing (2), because some cement has low pH. According to a dressing added with paraform, paraformaldehyde as a component has an effect of fixing pulp. However, it is difficult to say that it is fully safe to be applied in an oral cavity. Among those described in (1), a sodium fluoride solution and pasta added with sodium fluoride generally use 2% sodium fluoride (neutral). However, as it does not seal dentinal tubules, the effect of inhibiting hypersensitivity is very minor. When used for the purpose of protecting pulp, calcium hydroxide is effective. However, it has a very little effect on general dentinal hypersensitivity such as cold water pain. Ion introduction is a method of impregnating an ion tray in 2% sodium fluoride solution (neutral) and aggressively introducing fluoride with an aid of electric current, which requires an expensive device for introducing fluoride ions.
Further, according to (1) and (2), only a material containing effective components is applied on tooth surfaces, and therefore the effect is often temporary.
When the particle diameter of the material is smaller than diameter of dentinal tubules and the reaction between the two liquids occurs within the dentinal tubules to generate microparticle reaction products (that is, precipitates) within a short period of time, the dentinal tubules can be sealed. Further, when the microparticles evenly cover dentinal surface to seal the dentinal tubules, stimulation on the tubules is blocked, and therefore a huge therapeutic effect can be expected. To accomplish it, studies on physical properties of the material are required. In this regard, it is difficult to say that a sufficient improvement has been made with conventional materials.
The HY preparation (trade name: HYC) is mixture powder of tannin, zinc fluoride, strontium fluoride and zinc oxide. Although inhibition of hypersensitivity by an astringent effect of tannin and prevention of dental caries by fluoride are expected, as it instantly hardens in contact with water, it has a problem in handlability. There is also a problem that the cured product is colored with a dark color in an oral cavity.
Silver diamine fluoride preparation (trade name: SAFORIDE) can be easily coated on tooth surfaces and remain on the surfaces for a long period of time so that it has a superior effect of treating hypersensitivity and preventing secondary dental caries. However, due to precipitation of silver, coated tooth area is darkened like a black tooth. Thus, having a significant problem in aesthetics, its application is limited.
Regarding (3), a tooth neck not covered with enamel can be easily abraded by brushing and dentinal tubules are easily exposed in an abraded region to cause hypersensitivity. For other cases in which the dentinal tubules are exposed, the corresponding tooth area is filed and filled with cement or an adhesive resin to perform simultaneously the treatment of hypersensitivity and the restoration of a defective area. However, dentinal characteristics of the hypersensitivity include that no caries (soft dentin) are seen and a treatment without filing a healthy teeth is required.
Under the circumstances, presently, the strongest candidate of therapeutics for dentinal hypersensitivity is a method of using oxalate as reported in Patent Documents 6 and 7. 30% Aqueous solution of potassium oxalate provided by Protect, USA is clinically used as a therapeutic agent for dentinal hypersensitivity (Dentin Desensitizer) while a two-liquid set containing 30% aqueous solution of potassium oxalate and 3% aqueous solution of potassium hydrogen oxalate provided by O. P. Laboratories is also clinically used as a therapeutic agent for dentinal hypersensitivity (Dentin Desensitizer) (see for example, Patent Documents 6 and 7). However, none of them exhibits re-calcification or a prophylactic effect against secondary dental caries.
It is preferable that a treatment with a therapeutic agent for dentinal hypersensitivity is done within a short period of time, that is, it is washed with water within several tens of seconds after applied on tooth surfaces during dental treatment. Further, it preferably has not only an effect of inhibiting hypersensitivity but also an effect of preventing and treating secondary caries. Further, if there is no problem in terms of aesthetics, it can be expected to be used as a material which is applicable for a wide range of cases.
Meanwhile, a material allowing penetration of a mixture liquid into dentinal tubules or early caries lesions and production of nanoparticle precipitates within a short period of time, additionally comprising, in the precipitates, calcium phosphate or a fluoride compound based on expectation of obtaining calcification is desired. However, a material fully satisfying such conditions is not reported yet.
A dental composition for dentinal hypersensitivity comprising aqueous polymer emulsion particles, which have a smaller particle diameter than diameter of dentinal tubules so that it can react with a calcium compound and form a larger aggregate than the diameter of dentinal tubules, is suggested (Patent Document 5). However, it does not exhibit re-calcification or an effect of preventing secondary dental caries.
Pain may be caused by temperature stimulation or the like after having a dentin cavity or receiving a dental prosthetics. When dental caries are under progress or the like, a dentin cavity is formed as deep as near pulp, and therefore pain is often seen even after the treatment. To avoid it, a lining cement is placed on a dentin cavity surface close to pulp by using calcium hydroxide, glass ionomer cement, or the like. However, although having a high pulp protection effect, calcium hydroxide has no adhesiveness on teeth and is easily desorbed. Thus, an additional lining with other cements is required over the calcium hydroxide layer, and therefore it is laborious. The glass ionomer cement has problems such as discussed above.